Laser-guided weapons delivery systems include, inter alia, a direct-view optical telescope or `day`system used for daytime viewing and a thermal imaging or `night`system used for nighttime `viewing`. The day system is useful for viewing targets located remote from the system and is operative, via electro-optical systems, to facilitate aiming of the weapons system with which it is associated.
The thermal imaging system, used for facilitating aiming of the weapons system at night, is operative to `lock in` to thermal energy, typically infrared, emitted by the target. The thermal imaging system is mounted onto the optical telescope system such that the line of sight thereof is parallel to the line of sight of the thermal imaging system. The aligning of the line of sight of the thermal imaging system with that of the optical telescope system is known as `boresighting.`
In order to carry out boresighting, apparatus, whose use is well known to those skilled in the art, has been developed which employs an optical subsystem, removably mounted in association with the optical telescope system and the thermal imaging system. This optical subsystem is known as a boresight module.
A conventional boresight module typically comprises an aluminum housing having `day` and `night` optical windows and in which there is disposed an elongate quartz prism disposed along an axis and defining first and second parallel reflective surfaces transversely intersecting the axis and respectively associated with the day and night windows, a light source, a collimator for reflecting parallel to the axis nonparallel light rays projected at the collimator and a reticle, which is typically a movable arrangement of cross hairs.
Light rays are projected past the cross hairs and pass to the collimator and, as stated, are reflected therefrom parallel to the axis. If the collimator is not arranged along the axis then there is also provided a compensating prism to cause reflection of the rays parallel to the axis. The quartz prism is arranged such that some of the light rays are projected through the day window and some are projected through the night window.
The module is mounted in association with the optical telescope system and the thermal imaging system such that the respective day and night windows are aligned therewith and such that light rays projected through each of the windows pass into the respective optical system associated therewith. By techniques which are not the subject of the present invention, boresighting may thus be carried out.
As will be appreciated by those skilled in the art, precise alignment between the reflective surfaces and the collimator is crucial in weapons systems employing the above-described optical systems, an acceptable angular line of sight error in boresighting being typically in the order of one half of a degree or less.
A disadvantage of the above-described boresight module is that alignment errors may result from differential thermal expansion of different portions of the quartz prism and the aluminum housing in which it is located. Although the prism is generally mounted in the housing by kinematic mounts, it has been found that significant angular line of sight errors nonetheless occur, these errors being due, at least in part, to differential thermal expansion between opposing ends of the prism.
Disclosed in an article in SPIE Vol. 645 Optical Manufacturing, Testing and Aspheric Optics (1986) by Rudolf Hartmann and, (at the time of publication of the article) a subject of a patent application by the U.S. Army, Invention Ser. No. 07/411,413, is a boresight module constructed so as to eliminate differential thermal expansion of optical elements employed therein.
The boresight module to Hartmann comprises an elongate hollow prismatic element onto ends of which are molecularly bonded respective beamsplitter and annular reflecting plates, these plates being nominally parallel to each other and being associated with respective day and night optical windows as described above.
Among disadvantages inherent in the module described in the article by Hartmann is that, in order to eliminate differential thermal expansion, the reflecting plates are molecularly bonded onto the prismatic element. This adds significant time and expense to the manufacturing process. In addition, both the conventional boresight module as described earlier and that described by Hartmann constitute complex structures having relatively large numbers of components.